An oxygen sensor which detects an oxygen concentration in an exhaust gas and an air-fuel ratio sensor are known as a gas sensor which carries out a fuel efficiency improvement and combustion control of an internal combustion engine of an automobile or the like. Also, a reduction in the amount of nitrogen oxide (NOX) in the exhaust gas is required in keeping with a tightening of automobile exhaust gas regulations, and an NOX sensor which can directly measure an NOX concentration is being developed.
Each of these gas sensors has a gas sensor element including one or a plurality of cells, each of which has a pair of electrodes formed on the surface of an oxygen ion conducting solid electrolyte such as zirconia, and carries out a specified gas concentration detection based on an output from the gas sensor element.
Also, each of these gas sensors incorporates a heater for activating the solid electrolyte.
As these gas sensors, there is known a linear air-fuel ratio sensor (hereafter referred to also as a UEGO sensor) which, having two cells (an oxygen concentration sensing cell and an oxygen pump cell) disposed in such a way as to sandwich a measuring chamber, introduces a measured gas into the measuring chamber via a diffusion resistor, and detects oxygen contained in the measured gas. Furthermore, an NOX gas sensor in which a cell which detects an NOX gas concentration is disposed in addition to the two cells (the oxygen concentration sensing cell and the oxygen pump cell), and which has a total of three cells, is also known.
A sensor drive circuit is connected to this kind of gas sensor, and the gas sensor energizes the cells via the sensor drive circuit, measures a specified gas concentration in the measured gas based on outputs from the cells, and is called a gas sensor control apparatus, including the sensor drive circuit. Also, as an energization state in which the cells are energized, there are a protection energization state for protecting the gas sensor, a pre-activation energization state in which the gas sensor in a deactivated condition is energized with a minute current, a gas concentration measuring energization state for detecting a specified gas, and the like.
Among them, the protection energization state is such that, by electrically cutting off the continuity between the cells and sensor drive circuit, a current is prevented from flowing through the gas sensor, thus protecting the gas sensor. Also, the pre-activation energization state is a mode in which oxygen which is of a reference concentration is accumulated in a reference oxygen chamber of, for example, the oxygen concentration sensing cell by the cell being energized with a minute current, thus preparing for a gas concentration measurement.
However, it may happen that an anomaly, such as a short circuit with a battery or the ground, or a disconnection, occurs in a wire of the sensor drive circuit or gas sensor. Then, when the gas concentration measuring energization state in which a gas concentration is measured continues despite the fact that such a wire anomaly occurs, there is fear that an excessively high current flows through the gas sensor, and that the gas sensor is damaged.
For this kind of reason, a technology is being developed whereby, when a wire anomaly is detected, the connection of the sensor drive circuit side and gas sensor is electrically cut off, creating the protection energization state, and subsequently, anomaly contents and a site of occurrence of the anomaly are diagnosed (refer to Patent Document 1). Because of this, no more anomalous current continues to flow through the gas sensor, thus preventing the gas sensor being damaged.
Also, when a command to switch from another energization state to the gas concentration measuring energization state is output to the sensor drive circuit without recognizing a wire anomaly, there is fear that an excessively high current flows through the gas sensor, and that the gas sensor is damaged, in the same way.
For this kind of reason, a technology is being developed whereby a switching to the gas concentration measuring energization state is allowed only when the preceding state is the pre-activation energization state, thereby detecting an anomaly without damaging the gas sensor (refer to Patent Document 2). Because of this, in the event of the pre-activation energization state in which a minute current is caused to flow through the gas sensor even when a wire anomaly occurs, a voltage applied to the gas sensor departs from a normal range, meaning that it is possible to detect the wire anomaly.
Also, in the case of the heretofore known gas sensor control apparatus, a constant voltage source which applies a target voltage for controlling an Ip current flowing through the oxygen pump cell is provided in the sensor drive circuit, and the target voltage is supplied to a PID control circuit in the sensor drive circuit. Then, a configuration is such that an amount of deviation between an actual output voltage of the oxygen concentration sensing cell and the target voltage is PID computed by the PID control circuit in accordance with the oxygen concentration in the measuring chamber, and an Ip current in accordance with the amount of deviation flows through the oxygen pump cell.
Then, there is provided a Vp limiter which, when a terminal voltage of the oxygen pump cell exceeds a threshold value at which it becomes an overvoltage causing a blackening, changes the target voltage, and prevents the overvoltage from being applied.